For a superconducting wire for connection is used a superconducting fine multi-wire composed of a lamination of multiplicity of superconducting material wires which are embedded in a stabilizing material such as copper (Cu) and aluminum (Al) and stretched to a desired outer diameter. When such a superconducting wire is connected, various methods such as soldering, blazing, press bonding and welding have been conventionally adopted, but the connecting portion obtained by any of these methods has a large electric resistance and generates a large amount of heat during electric conduction, which produces problems in practical use.
In order to ameliorate these defects, a method of connecting a superconducting fine multi-wire by accommodating the fine multi-wire in a connecting tube with the exposed superconducting material wires laminated on each other and press bonding through the connecting tube for the purpose of establishing electric conduction is disclosed in Japanese Patent Laid-Open No. 16207/1984. In this method, the stabilizing member is removed from the connecting portion of the superconducting material wires and the exposed superconducting material wires were accommodated in the connecting pipe in the state of being laminated on each other. The superconducting filaments were press bonded through the pipe so as to bond the superconducting filaments accommodated with each other.
In this connecting method, however, the superconducting filaments are brought into contact with each other only on the portions at which the outer surfaces thereof are, thereby making it difficult to secure a high critical current value. In addition, since the filaments are press bonded only in one direction, the contact between the superconducting filaments is insufficient.
Japanese Patent Laid-open No. 234880/1987 proposes a method of bonding a superconducting wire which is capable of enhancing the packing ratio of the superconducting filaments. This method is characterized in that each of the exposed core filament is sandwiched between a double core having exposed connecting superconducting filaments, the sandwiched portions of all the filaments are integrally covered with a metal ring and the filaments are bonded by press bonding the metal ring. For example, the end portion of each connecting superconducting filament is extended so as to cover the end of the stabilizing member and the end portion of the metal ring is situated at the end portion of each filament. In this state, the metal ring is press bonded so as to bond the filaments. As in this example, in the case in which the number of superconducting material wires (filaments) is very small, it is effective to the enhancement of the packing ratio to use connecting superconducting filaments. However, since the filaments are press bonded only in one direction, the contact between the superconducting filaments cannot be said to be sufficient. In addition, the sectional area or the length of the stabilizing member after press bonding is not taken into consideration, and there is variation in the connection resistance value. Thus, the superconducting wire obtained by this method cannot be used as a permanent current superconductor.
In the above-described prior art, since neither a method of press bonding the superconducting material wires (filaments) by the stabilizing member nor the sectional areal ratio or the length of the connecting portion is taken into consideration, the real connection of the superconducting material wires cannot be achieved, which would produce a problem in the permanent electrical characteristics. In other words, the prior art has mainly aimed at combining the superconducting material wires and cannot realize the connection which can exert the proximity effect of the superconducting wire materials or prevent the shunt loss.